A study is performed on a sample of residential homes to discover whether the size of the monthly heating bill depends on the type of fuel or the type of home. In particular, three types of fuel are examined: electric, natural gas, and oil. Also, all homes are classified into two types: those on a single level and those with at least two stories.

What might an interaction effect look like in this situation? Intuitively, do you think there is any reason to expect an interaction effect?

Your responses are to be in detail with examples, citations with references. Be sure to identify the source of your example in your posting.